Annular or spherical blowout preventers (BOPs) are large, hydraulically operated valves used in the oil and gas industry to control a wellbore at the ground level. Annular BOPs primarily function to seal or close the wellbore, when actuated, to thereby prevent a blowout.
Structurally, annular BOPS comprise a body formed by mating upper and lower housings that enclose a deformable, resilient packing element for sealing its central bore which connects with the wellbore. The body further encloses a hydraulically operated piston, which actuates the opening and closing of the resilient packing element.
Contamination of the BOP hydraulic fluid system, by fluid and/or debris from the wellbore, needs to be prevented in order for the BOP to function properly. Various types of pressure seals are used to provide a physical separation between the fluids in the hydraulic fluid chamber of the BOP and its central bore. The prior art pressure seals conventionally used in this service, however, have not satisfactorily withstood the damage and degradation that occurs over time as a result of exposure to contaminants in the wellbore fluid. In addition, such common pressure seals are of a size and shape that they can roll or twist in use, thereby releasing their sealed engagement and allowing leakage of fluid. Accordingly, frequent inspection, repair and replacement of the common pressure seals is required in order to prevent the leakage of wellbore fluid into the hydraulic fluid system of the BOP.
One attempt to solve this problem has been to utilize what is known as a single, dual-functioning wiper seal, which serves to provide:                a) a pressure seal for preventing leakage of fluid from the wellbore into the hydraulic fluid system of the BOP, and        b) a wiper seal for preventing dirt and debris from the fluid in the wellbore from entering the hydraulic fluid system.        
Dual-functioning seals, however, are not effective pressure seals and are still prone to degradation over time. Frequent repair and/or replacement of dual-functioning wiper seals continues to pose a problem for BOP manufacturers and operators in the oil and gas industry.
There is therefore a need for modified annular seal assembly that is designed to effectively seal the interface between the hydraulic fluid system of a BOP and the fluid within the wellbore, while at the same time resisting damage cause by wellbore fluid contaminants.